Easyjet, Ryanair, Eurostar, weekends in Europe with a Bicycle from UK

I'm living in London and so often when i'm off in Europe for a few days in say Paris or Berlin I wish I could take my bicycle.

Has anyone had experience taking a fullsize or folder on budget airlines such as Ryanair, Easyjet, BMIbaby, etc..?

I have hired bikes in Copenhagen which was loads of fun and used the velib system in Paris and the DB bikes in Berlin but its always a bit inconvenient to find / hire a bike or get it back in 30 mins. and the DB in Berlin is just insanely expensive and inconvenient with calling up constantly to get codes / drop-off bikes.

So sometimes I just wish I had my own to cruise around on a lock-up where I go. but is it too much hasle/risk/expense to take for a short 2-3 night trip?

Full size bikes need a fair amount of preparation to travel by budget airlines. I do the usual lower saddle, flip bars, remove pedals, cover all tubes with plumbing insulation and seal in a tough plastic bag. I have to deal with the packing material and carry all the tools and accessories I need.
For a weekend, its not worth the hassle, and you also give up the freedom travelling with only hand luggage.
It might be viable with a simple folding bike that you can put inside a case (and a case that you can carry when you ride).